Magnetometers are devices used to measure magnetic fields. Magnetometers based on superconducting quantum interference devices (SQUIDs) are among the most sensitive devices for measuring small magnetic fields. A conventional DC SQUID consists of two parallel Josephson Junctions disposed along a superconducting loop. These devices convert magnetic flux threading the superconducting loop into a quantity (e.g., current or voltage) that may be measured by an associated electronic stage.
Recently, advances in the field of nanotechnology have driven the search for SQUID magnetometers capable of detecting magnetic flux changes associated with nanoscale molecules and objects. See John Gallop, Superconductor Science and Technology, 16, 1575-1582 (2003) and Ling Hao et al., Superconductor Science and Technology, 16, 1479-1482 (2003). In one instance, a SQUID loop smaller than 1 μm has been used to measure the flipping of ˜1000 electron spins. See M. Jamet, W. Wernsdorfer, C. Thirion, D. Mailly, V. Dupuis, P. Mélinon and A. Péres, Physical Review Letters, 86, 4676 (2001). Unfortunately, the sensitivity of SQUID magnetometers has been limited by the residual noise in these devices.
At low temperatures, the limiting noise is thought to be the circulating noise currents in the SQUID loop. These noise currents would prevent the noise energy in a SQUID from being smaller than /2, where h=2 π is Planck's constant, as discussed in John Gallop, Superconductor Science and Technology, 16, 1575-1582 (2003) and Ling Hao et al., Superconductor Science and Technology, 16, 1479-1482 (2003). This means that a change in magnetic field associated with an energy change of 0.5×10−34 J is the minimum change that could be detected by a SQUID in a 1 Hz bandwidth. A bandwidth of 1 Hz corresponds approximately to a measurement time of 1 second. In practice that measurements longer than 1 second do not improve the sensitivity, as 1/f noise becomes larger.
In theory, an ideal SQUID magnetometer could be used to measure the change in magnetic field associated with a change in the nuclear spin of a single proton. In a strong magnetic field, the nuclear spin of a proton is in one of two states, separated by an energy difference ofΔE=γB,
where B is the applied magnetic field and the proton has a gyromagnetic ratio of γ=26.75×107 rad s−1 T−1. Increasing B increases the energy difference, ΔE, but SQUIDs cannot function in magnetic fields that are too large. Niobium SQUIDs can be used at ˜0.01 T, where the energy difference of a proton spin flip is ΔE≈2.7×10−28 J. See, for example, Tsuyoshi Tajima, Proceedings of 8th European Particle Accelerator Conference, http://apt.lanl.gov/documents/pdf/LA-UR-02-3042.pdf; E. M. Forgan, S. J. Levett, P. G. Kealey, R. Cubitt, C. D. Dewhurst and D. Fort, Physical Review Letters, 88, 167003, 2002; and H. R. Kerchner, D. K. Christen and S. T. Sekula, Physical Review B, 21, 86 (1980). The highest coupling of a spin to the SQUID is achieved when the spin lies on the Josephson Junction. In this case, up to half of the magnetic flux is coupled, so the maximum energy detected by the SQUID as a result of the spin flip would fall to 1.3×10−28 J. Under these ideal conditions, a SQUID could be used to measure a single nuclear spin flip, with a signal to noise ratio (SNR) of >106.
SQUIDs have been reported with a noise energy of 3 . See, for example, D. J. Van Harlingen, R. H. Koch and J. Clarke, App. Phys. Lett. 41, 197 (1982). However, a need exists for a SQUID magnetometer in which there is good coupling between the flux from a magnetic particle and the magnetometer. When this improved coupling is achieved, the need arises to increase the SQUID sensitivity further, for demanding applications such as single-molecule NMR.